1. Two endpoints of a circle with any diameter are divided into two arcs, and each arc is called a semicircle.

2. Vertical diameter theorem: the diameter perpendicular to the chord bisects the chord and bisects the arc opposite to the chord.

3. A circle is an axisymmetric figure, and every straight line passing through the center of the circle is its axis of symmetry.

4. In the same circle or circle, the isocentric angle has equal arc, chord and chord center distance.

5. Tangent length theorem: the tangent lengths of two tangents drawn from a point outside the circle are equal, and the connecting line between the center of the circle and this point bisects the included angle of the two tangents.